Displays, such as liquid crystal (LC) and electrophoretic displays include particles suspended in a medium sandwiched between a drive or pixel electrode and a common electrode. The pixel electrode includes pixel drivers, such as an array of thin film transistors (TFTs) that are controlled to switch on and off to form an image on the display. The voltage difference (VDE=VEink=VCE−Vpx as shown in FIGS. 3 and 5A) between a TFT(s) or the pixel electrode(s) and the common electrode, which is on the viewer's side of the display, causes migration of the suspended particles, thus forming the image. Displays with an array of individually controlled TFTs or pixels are referred to as active-matrix displays.
In order to change image content on an electrophoretic display, such as from E Ink Corporation for example, new image information is written for a certain amount of time, such as 500 ms to 1000 ms. As the refresh rate of the active-matrix is usually higher, this results in addressing the same image content during a number of frames, such as at a frame rate of 50 Hz, 25 to 50 frames. Circuitry to drive displays, as well as electrophoretic displays, are well known, such as described in U.S. Pat. No. 5,617,111 to Saitoh, International Publication No. WO 2005/034075 to Johnson, International Publication No. WO 2005/055187 to Shikina, U.S. Pat. No. 6,906,851 to Yuasa, and U.S. Patent Application Publication No. 2005/0179852 to Kawai, each of which is incorporated herein by reference in its entirety.
FIG. 1 shows a schematic representation 100 of the E-ink principle, where different color particles, such as black micro-particles 110 and white micro-particles 120 suspended in a medium 130, are encapsulated by the wall of an E-ink capsule 140. Typically, the E-ink capsule 140 has a diameter of approximately 200 microns. A voltage source 150 is connected across a pixel electrode 160 and a common electrode 170 located on the side of the display viewed by a viewer 180. The voltage on the pixel electrode 160 is referred to as the pixel voltage Vpx, while the voltage on the common electrode 170 is referred to as the common electrode voltage VCE. The voltage across the pixel or capsule 140, i.e., the difference between the common electrode and pixel voltages, is shown in FIG. 5A as VEink.
Addressing of the E-ink 140 from black to white, for example, requires a pixel represented as a display effect or pixel capacitor CDE in FIGS. 3 and 5A and connected between pixel electrodes 160 and a common electrode 170, to be charged to −15V during 500 ms to 1000 ms. That is, the pixel voltage Vpx at the pixel electrode 160 (also shown in FIG. 5A as the voltage at node P) is charged to −15V, and VEink=VCE−Vpx=0−(−15)=+15V. During this time, the white particles 120 drift towards the top common electrode 170, while the black particles 110 drift towards the bottom (active-matrix, e.g., TFT, back plane) pixel electrode 160, also referred to as the pixel pad.
Switching to a black screen, where the black particles 110 move towards the common electrode 170, requires a positive pixel voltage Vpx the pixel electrode 160 with respect to the common electrode voltage VCE. In the case where VCE=0V and Vpx=+15V, the voltage across the pixel (CDE in FIG. 5A) is VEink=VCE−Vpx=0−(+15)=−15V. When the voltage across the pixel VEink is 0V, such as when both the pixel voltage Vpx at the pixel electrode 160 and the common electrode voltage VCE are 0V (Vpx=VcE=0), then the E-ink particles 110. 120 do not switch or move. As shown in the graph 200 of FIG. 2, the switching time of the E-ink 140 (or CDE in FIGS. 3 and 5A) to switch between the black and white states decreases (i.e., the switching speed increases or is faster) with increasing voltage across the pixel VDE or VEink. The graph 200, which shows the voltage across the pixel VEink on the y-axis in volts versus time in seconds, applies similarly to both switching from 95% black to 95% white screen state, and vice verse. It should be noted that the switching time decreases by more than a factor two when the drive voltage is doubled. The switching speed therefore increases super-linear with the applied drive voltage.
FIG. 3 shows the equivalent circuit 300 for driving a pixel (e.g., capsule 140 in FIG. 1) in an active-matrix display that includes a matrix or array 400 of cells that include one transistor 310 per cell or pixel (e.g., pixel capacitor CDE) as shown in FIG. 4. A row of pixels is selected by applying the appropriate select voltage to the select line or row electrode 320 connecting the TFT gates for that row of pixels. When a row of pixels is selected, a desired voltage may be applied to each pixel via its data line or the column electrode 330. When a pixel is selected, it is desired to apply a given voltage to that pixel alone and not to any non-selected pixels. The non-selected pixels should be sufficiently isolated from the voltages circulating through the array for the selected pixels. External controller(s) and drive circuitry is also connected to the cell matrix 400. The external circuits may be connected to the cell matrix 400 by flex-printed circuit board connections, elastomeric interconnects, tape-automated bonding, chip-on-glass, chip-on-plastic and other suitable technologies. Of course, the controllers and drive circuitry may also be integrated with the active matrix itself.
In FIG. 4, the common electrodes 170 are connected to ground instead of a voltage source that provide VCE. The transistors 310 may be TFTs, for example, which may be MOSFET transistors 310, as shown in FIG. 3, and are controlled to turn ON/OFF (i.e., switch between a conductive state, where current Id flows between the source S and drain D, and non-conductive state) by voltage levels applied to row electrodes 320 connected to their gates G, referred to as Vrow or Vgate. The sources S of the TFTs 310 are connected to column electrodes 330 where data or image voltage levels, also referred to as the column voltage Vcol are applied.
As shown in FIG. 3, various capacitors are connected to the drain of the TFT 310, namely, the display effect capacitor CDE that contains the display effect also referred to as the pixel capacitor, and a gate-drain parasitic capacitor Cgd between the TFT gate G and drain D shown in dashed lines in FIG. 3. In order to hold the charge or maintain the level of pixel voltage Vpx (at node P to remain close to the level of the column voltage Vcol) between two select or TFT-ON states (as shown by reference numeral 765 in FIG. 7), a storage capacitor Cst may be provided between the TFT drain D and a storage capacitor line 340. Instead of the separate storage capacitor line 340, it is also possible to use the next or the previous row electrode as the storage capacitor line.
Conventional active matrix E-ink displays suffer from various drawbacks. One drawback is that power consumption during an image update is relatively large, due to the relatively high voltages that must be applied during addressing of the display. A straightforward solution would be lowering the addressing voltages. However, the disadvantage of the lower voltage levels is that the image update time increases more than linear with the voltage reduction as shown in FIG. 2, leading to very long image update times (i.e., slower image updates). Another drawback is that the image update time of E-ink is relatively long despite the high voltage levels. Accordingly, there is a need for better displays, such as displays with decreased image update time without an increase in the addressing voltage and thus without an increase of power consumption.